Conventionally, a fuel cell system is known to have: a radiator disposed in a circulating circuit of coolant that cools a fuel cell unit; a bypass passage connecting an upstream side of the radiator of the circulating circuit and an downstream side of the radiator of the circulating circuit so as to make coolant bypasses the radiator; a pump device arranged downstream of a connecting point where the bypass passage is connected to the circulating circuit; and a three-way valve device that is disposed at the connecting point where the bypass passage is connected to the circulating circuit so as to adjust a flow ratio between coolant flowing through the radiator and coolant flowing through the bypass passage (see Patent Document 1).